During the life of an airplane there are occasions when the skin of the plane is damaged and must be repaired. Present techniques include fastening a sheet of new skin, commonly referred to as a doubler, on the damaged area. This is usually accomplished by removing the fasteners in the original skin, placing the doubler over the skin, drilling a hole through the doubler and inserting a new fastener into the skin and doubler.
The outside surface of the original skin usually has a countersunk hole to allow a countersunk fastener to fit flush with the surface of the skin. When a doubler is added and the new fastener is inserted, the countersink leaves a space between the original skin and the new fastener. This space degrades the structural integrity of the airplane skin. To eliminate this space a washer is bonded into the countersink. The doubler is then placed over the skin and a new larger hole is drilled through the skin, washer and doubler and a new fastener is pressed into the hole. If the bond between the washer and the skin is not strong enough, the washer will break away and spin when drilling is attempted. When this occurs the drill is unable to penetrate the washer and doubler, and the washer must be taken out. To replace the washer, the whole doubler must be removed, a process that is costly and time consuming if other fasteners have already been installed in the doubler.
One solution has been to put in a washer with an inner hole diameter larger than the drill, to allow the drill to go through the washer and into the doubler without interference from the washer. But the oversized inner hole again leaves a space between the new fastener and the washer increasing the stress on the fastener.